50 hp johnson bigger prop

[ronward]

While tuning my motor (50ESL74M) I accidentally hit the gear lever and threw it in reverse in the water barrel. Needless to say, prop and barrel are no longer useable. I have a replacement but it is bigger. About 1" more dia. but appears to be same pitch. Hypothetically, how would this prop act in the water on a boat rated for a 50 horse? This motor is on a stand right now and I don't know what I will put it on, I just want a generic opinion of what the larger diameter might do. Lookin at a 17' tri-hull, pretty light, but deep V. Whatcha think?

 

[Bob_VT]

Re: 50 hp johnson bigger prop

Larger diameter will drop your rpms a bit. Your true test will be in the open water.

 

[ronward]

Re: 50 hp johnson bigger prop

I don't really understand pitch vs. diameter. Can someone help?

 

[ronward]

Re: 50 hp johnson bigger prop

I know I am considering putting a too small motor on this boat, but it is what I have to work with. Can, and how, can this motor push this boat. Speed is not an issue, only planing for the most I can get from this setup. If that is even possible

 

[steelespike]

Re: 50 hp johnson bigger prop

Diameter is a secondary consideration usually diameter follows pitch change: as pitch goes down diameter usually goes up.
With the right prop your motor can manage a 17 ft glass boat; but the correct pitch will be critical.
Your prop numbers should be near the base of the blades or under the prop nut.
Its very important that the motor be able to operate within its rated rpm range.
If you read the stickys at the top of the prop forum it will help.
You will need to do some lightly loaded test runs get us the prop size and the wot rpm and gps speed.
Once it is setup to operate close to its max rated rpm you can operate it at what ever throttle setting you want.
You very likely will need to change to a lower pitch.

 

[torbjorn]

Re: 50 hp johnson bigger prop

Necessary pitch to push the rig at a certain speed at a given RPM can be estimated. The diameter necessary
to absorb the hp at the propshaft can be calculated as well. The lower the gear ratio the larger the diameter
needed. Hence, 13 1/2" dia. on a Johnson 45 commercial model with 8/3 gearing, and 8 1/2" dia. on a 60 hp
OMC racing motor (the same block) with 1:1 gearing. You want to run the lowest dia. you can get away with in order
to get the RPM OMC outboards, 6000 RPM. And always run Evinrude or Mercury oil, avoid the cheap stuff or
most other oils like the plague (Amsoil is also good).

I don't really understand pitch vs. diameter. Can someone help?

 

[jestor68]

Re: 50 hp johnson bigger prop

Some professors say an inch of diameter is equal to an inch of pitch; while one I've talked to feels it's more like 2 inches of pitch. I have always subscribed to the 1 to 1 relationship.

Contrary to what a lot of folks apparently think, the diameter size doesn't just magically happen. It is carefully matched to the shaft speed(gear ratio).

There should be a size stamped somewhere on the prop's hub.

While you should be able to run something like 14" dia props on a "large" gear case, I suspect the pitch will need to be fairly low to push a 17 footer with a 50 hp.

With a accurate weight of the boat, you can run the numbers through one of the prop selectors and get a good idea which prop size you'd need; along with the educated guesses of the experienced prop folks that hang around here.

 

[ronward]

Re: 50 hp johnson bigger prop

I put the 17 footer on the water yesterday. Was very surprised that it got on plane quite easy. Not real fast but it works. Problem is (as was suggested on here) the rpm's are way down. I'm thinking about a 14x13 prop as a starting point. Any better ideas? I'm sorry I don't have more info to give. I can't find any numbers on prop. Old prop was 13 1/4x17 originally on 14' runabout. New boat is actually 16 1/2'.

 

[steelespike]

Re: 50 hp johnson bigger prop

So you're saying the prop is a 17"?
With basically no information There's no way to prop it right and to be sure it is right.
A 15" is reasonable but you should get a tach and a gps for speed and do a lightly loaded test run.
Its not a stretch that without the proper information you could end up buying 3 props to get one that is right.
At least get us a good gps speed. Some cells have a gps app.

 

[Silvertip]

Re: 50 hp johnson bigger prop

If that engine has a 2:34 gear ratio, the "typical" prop sizes for a planning hull the size of your boat would be 11-3/4 x 17P or 12-1/4 x 15P. A 14 inch diameter prop is way too big and is part of the reason the rpm is way down. I've done extensive testing of this motor and almost always go back to a 12-1/4 x 15P cupped prop (aluminum for cost purposes) for optimum performance. An uncupped prop of 11-3/4 x 17P provides almost identical performance.